LCOV - code coverage report
Current view: top level - lib/CodeGen - ShadowStackGCLowering.cpp (source / functions) Hit Total Coverage
Test: llvm-toolchain.info Lines: 106 111 95.5 %
Date: 2018-05-20 00:06:23 Functions: 13 13 100.0 %
Legend: Lines: hit not hit

          Line data    Source code
       1             : //===- ShadowStackGCLowering.cpp - Custom lowering for shadow-stack gc ----===//
       2             : //
       3             : //                     The LLVM Compiler Infrastructure
       4             : //
       5             : // This file is distributed under the University of Illinois Open Source
       6             : // License. See LICENSE.TXT for details.
       7             : //
       8             : //===----------------------------------------------------------------------===//
       9             : //
      10             : // This file contains the custom lowering code required by the shadow-stack GC
      11             : // strategy.
      12             : //
      13             : // This pass implements the code transformation described in this paper:
      14             : //   "Accurate Garbage Collection in an Uncooperative Environment"
      15             : //   Fergus Henderson, ISMM, 2002
      16             : //
      17             : //===----------------------------------------------------------------------===//
      18             : 
      19             : #include "llvm/ADT/SmallVector.h"
      20             : #include "llvm/ADT/StringExtras.h"
      21             : #include "llvm/CodeGen/Passes.h"
      22             : #include "llvm/IR/BasicBlock.h"
      23             : #include "llvm/IR/Constant.h"
      24             : #include "llvm/IR/Constants.h"
      25             : #include "llvm/IR/DerivedTypes.h"
      26             : #include "llvm/IR/Function.h"
      27             : #include "llvm/IR/GlobalValue.h"
      28             : #include "llvm/IR/GlobalVariable.h"
      29             : #include "llvm/IR/IRBuilder.h"
      30             : #include "llvm/IR/Instructions.h"
      31             : #include "llvm/IR/IntrinsicInst.h"
      32             : #include "llvm/IR/Intrinsics.h"
      33             : #include "llvm/IR/Module.h"
      34             : #include "llvm/IR/Type.h"
      35             : #include "llvm/IR/Value.h"
      36             : #include "llvm/Pass.h"
      37             : #include "llvm/Support/Casting.h"
      38             : #include "llvm/Transforms/Utils/EscapeEnumerator.h"
      39             : #include <cassert>
      40             : #include <cstddef>
      41             : #include <string>
      42             : #include <utility>
      43             : #include <vector>
      44             : 
      45             : using namespace llvm;
      46             : 
      47             : #define DEBUG_TYPE "shadow-stack-gc-lowering"
      48             : 
      49             : namespace {
      50             : 
      51       66093 : class ShadowStackGCLowering : public FunctionPass {
      52             :   /// RootChain - This is the global linked-list that contains the chain of GC
      53             :   /// roots.
      54             :   GlobalVariable *Head = nullptr;
      55             : 
      56             :   /// StackEntryTy - Abstract type of a link in the shadow stack.
      57             :   StructType *StackEntryTy = nullptr;
      58             :   StructType *FrameMapTy = nullptr;
      59             : 
      60             :   /// Roots - GC roots in the current function. Each is a pair of the
      61             :   /// intrinsic call and its corresponding alloca.
      62             :   std::vector<std::pair<CallInst *, AllocaInst *>> Roots;
      63             : 
      64             : public:
      65             :   static char ID;
      66             : 
      67             :   ShadowStackGCLowering();
      68             : 
      69             :   bool doInitialization(Module &M) override;
      70             :   bool runOnFunction(Function &F) override;
      71             : 
      72             : private:
      73             :   bool IsNullValue(Value *V);
      74             :   Constant *GetFrameMap(Function &F);
      75             :   Type *GetConcreteStackEntryType(Function &F);
      76             :   void CollectRoots(Function &F);
      77             : 
      78             :   static GetElementPtrInst *CreateGEP(LLVMContext &Context, IRBuilder<> &B,
      79             :                                       Type *Ty, Value *BasePtr, int Idx1,
      80             :                                       const char *Name);
      81             :   static GetElementPtrInst *CreateGEP(LLVMContext &Context, IRBuilder<> &B,
      82             :                                       Type *Ty, Value *BasePtr, int Idx1, int Idx2,
      83             :                                       const char *Name);
      84             : };
      85             : 
      86             : } // end anonymous namespace
      87             : 
      88             : char ShadowStackGCLowering::ID = 0;
      89             : 
      90       21819 : INITIALIZE_PASS_BEGIN(ShadowStackGCLowering, DEBUG_TYPE,
      91             :                       "Shadow Stack GC Lowering", false, false)
      92       21819 : INITIALIZE_PASS_DEPENDENCY(GCModuleInfo)
      93       87909 : INITIALIZE_PASS_END(ShadowStackGCLowering, DEBUG_TYPE,
      94             :                     "Shadow Stack GC Lowering", false, false)
      95             : 
      96       22136 : FunctionPass *llvm::createShadowStackGCLoweringPass() { return new ShadowStackGCLowering(); }
      97             : 
      98       44272 : ShadowStackGCLowering::ShadowStackGCLowering() : FunctionPass(ID) {
      99       22136 :   initializeShadowStackGCLoweringPass(*PassRegistry::getPassRegistry());
     100       22136 : }
     101             : 
     102           2 : Constant *ShadowStackGCLowering::GetFrameMap(Function &F) {
     103             :   // doInitialization creates the abstract type of this value.
     104           2 :   Type *VoidPtr = Type::getInt8PtrTy(F.getContext());
     105             : 
     106             :   // Truncate the ShadowStackDescriptor if some metadata is null.
     107             :   unsigned NumMeta = 0;
     108             :   SmallVector<Constant *, 16> Metadata;
     109          13 :   for (unsigned I = 0; I != Roots.size(); ++I) {
     110           3 :     Constant *C = cast<Constant>(Roots[I].first->getArgOperand(1));
     111           3 :     if (!C->isNullValue())
     112           0 :       NumMeta = I + 1;
     113           3 :     Metadata.push_back(ConstantExpr::getBitCast(C, VoidPtr));
     114             :   }
     115           2 :   Metadata.resize(NumMeta);
     116             : 
     117           2 :   Type *Int32Ty = Type::getInt32Ty(F.getContext());
     118             : 
     119             :   Constant *BaseElts[] = {
     120           4 :       ConstantInt::get(Int32Ty, Roots.size(), false),
     121           2 :       ConstantInt::get(Int32Ty, NumMeta, false),
     122           4 :   };
     123             : 
     124             :   Constant *DescriptorElts[] = {
     125           4 :       ConstantStruct::get(FrameMapTy, BaseElts),
     126           4 :       ConstantArray::get(ArrayType::get(VoidPtr, NumMeta), Metadata)};
     127             : 
     128           2 :   Type *EltTys[] = {DescriptorElts[0]->getType(), DescriptorElts[1]->getType()};
     129           8 :   StructType *STy = StructType::create(EltTys, "gc_map." + utostr(NumMeta));
     130             : 
     131           2 :   Constant *FrameMap = ConstantStruct::get(STy, DescriptorElts);
     132             : 
     133             :   // FIXME: Is this actually dangerous as WritingAnLLVMPass.html claims? Seems
     134             :   //        that, short of multithreaded LLVM, it should be safe; all that is
     135             :   //        necessary is that a simple Module::iterator loop not be invalidated.
     136             :   //        Appending to the GlobalVariable list is safe in that sense.
     137             :   //
     138             :   //        All of the output passes emit globals last. The ExecutionEngine
     139             :   //        explicitly supports adding globals to the module after
     140             :   //        initialization.
     141             :   //
     142             :   //        Still, if it isn't deemed acceptable, then this transformation needs
     143             :   //        to be a ModulePass (which means it cannot be in the 'llc' pipeline
     144             :   //        (which uses a FunctionPassManager (which segfaults (not asserts) if
     145             :   //        provided a ModulePass))).
     146           2 :   Constant *GV = new GlobalVariable(*F.getParent(), FrameMap->getType(), true,
     147             :                                     GlobalVariable::InternalLinkage, FrameMap,
     148           4 :                                     "__gc_" + F.getName());
     149             : 
     150             :   Constant *GEPIndices[2] = {
     151           2 :       ConstantInt::get(Type::getInt32Ty(F.getContext()), 0),
     152           2 :       ConstantInt::get(Type::getInt32Ty(F.getContext()), 0)};
     153           4 :   return ConstantExpr::getGetElementPtr(FrameMap->getType(), GV, GEPIndices);
     154             : }
     155             : 
     156           2 : Type *ShadowStackGCLowering::GetConcreteStackEntryType(Function &F) {
     157             :   // doInitialization creates the generic version of this type.
     158             :   std::vector<Type *> EltTys;
     159           4 :   EltTys.push_back(StackEntryTy);
     160          13 :   for (size_t I = 0; I != Roots.size(); I++)
     161           6 :     EltTys.push_back(Roots[I].second->getAllocatedType());
     162             : 
     163          10 :   return StructType::create(EltTys, ("gc_stackentry." + F.getName()).str());
     164             : }
     165             : 
     166             : /// doInitialization - If this module uses the GC intrinsics, find them now. If
     167             : /// not, exit fast.
     168       22023 : bool ShadowStackGCLowering::doInitialization(Module &M) {
     169             :   bool Active = false;
     170      295715 :   for (Function &F : M) {
     171      547388 :     if (F.hasGC() && F.getGC() == std::string("shadow-stack")) {
     172             :       Active = true;
     173             :       break;
     174             :     }
     175             :   }
     176       22023 :   if (!Active)
     177             :     return false;
     178             :   
     179             :   // struct FrameMap {
     180             :   //   int32_t NumRoots; // Number of roots in stack frame.
     181             :   //   int32_t NumMeta;  // Number of metadata descriptors. May be < NumRoots.
     182             :   //   void *Meta[];     // May be absent for roots without metadata.
     183             :   // };
     184             :   std::vector<Type *> EltTys;
     185             :   // 32 bits is ok up to a 32GB stack frame. :)
     186           4 :   EltTys.push_back(Type::getInt32Ty(M.getContext()));
     187             :   // Specifies length of variable length array.
     188           4 :   EltTys.push_back(Type::getInt32Ty(M.getContext()));
     189           2 :   FrameMapTy = StructType::create(EltTys, "gc_map");
     190             :   PointerType *FrameMapPtrTy = PointerType::getUnqual(FrameMapTy);
     191             : 
     192             :   // struct StackEntry {
     193             :   //   ShadowStackEntry *Next; // Caller's stack entry.
     194             :   //   FrameMap *Map;          // Pointer to constant FrameMap.
     195             :   //   void *Roots[];          // Stack roots (in-place array, so we pretend).
     196             :   // };
     197             : 
     198           2 :   StackEntryTy = StructType::create(M.getContext(), "gc_stackentry");
     199             : 
     200             :   EltTys.clear();
     201           4 :   EltTys.push_back(PointerType::getUnqual(StackEntryTy));
     202           4 :   EltTys.push_back(FrameMapPtrTy);
     203           4 :   StackEntryTy->setBody(EltTys);
     204           2 :   PointerType *StackEntryPtrTy = PointerType::getUnqual(StackEntryTy);
     205             : 
     206             :   // Get the root chain if it already exists.
     207           2 :   Head = M.getGlobalVariable("llvm_gc_root_chain");
     208           2 :   if (!Head) {
     209             :     // If the root chain does not exist, insert a new one with linkonce
     210             :     // linkage!
     211           2 :     Head = new GlobalVariable(
     212             :         M, StackEntryPtrTy, false, GlobalValue::LinkOnceAnyLinkage,
     213           4 :         Constant::getNullValue(StackEntryPtrTy), "llvm_gc_root_chain");
     214           0 :   } else if (Head->hasExternalLinkage() && Head->isDeclaration()) {
     215           0 :     Head->setInitializer(Constant::getNullValue(StackEntryPtrTy));
     216           0 :     Head->setLinkage(GlobalValue::LinkOnceAnyLinkage);
     217             :   }
     218             : 
     219             :   return true;
     220             : }
     221             : 
     222             : bool ShadowStackGCLowering::IsNullValue(Value *V) {
     223             :   if (Constant *C = dyn_cast<Constant>(V))
     224           3 :     return C->isNullValue();
     225             :   return false;
     226             : }
     227             : 
     228           2 : void ShadowStackGCLowering::CollectRoots(Function &F) {
     229             :   // FIXME: Account for original alignment. Could fragment the root array.
     230             :   //   Approach 1: Null initialize empty slots at runtime. Yuck.
     231             :   //   Approach 2: Emit a map of the array instead of just a count.
     232             : 
     233             :   assert(Roots.empty() && "Not cleaned up?");
     234             : 
     235             :   SmallVector<std::pair<CallInst *, AllocaInst *>, 16> MetaRoots;
     236             : 
     237           6 :   for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
     238          31 :     for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;)
     239             :       if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++))
     240             :         if (Function *F = CI->getCalledFunction())
     241           3 :           if (F->getIntrinsicID() == Intrinsic::gcroot) {
     242             :             std::pair<CallInst *, AllocaInst *> Pair = std::make_pair(
     243             :                 CI,
     244           3 :                 cast<AllocaInst>(CI->getArgOperand(0)->stripPointerCasts()));
     245           3 :             if (IsNullValue(CI->getArgOperand(1)))
     246           3 :               Roots.push_back(Pair);
     247             :             else
     248           0 :               MetaRoots.push_back(Pair);
     249             :           }
     250             : 
     251             :   // Number roots with metadata (usually empty) at the beginning, so that the
     252             :   // FrameMap::Meta array can be elided.
     253           2 :   Roots.insert(Roots.begin(), MetaRoots.begin(), MetaRoots.end());
     254           2 : }
     255             : 
     256           6 : GetElementPtrInst *ShadowStackGCLowering::CreateGEP(LLVMContext &Context,
     257             :                                                     IRBuilder<> &B, Type *Ty,
     258             :                                                     Value *BasePtr, int Idx,
     259             :                                                     int Idx2,
     260             :                                                     const char *Name) {
     261           6 :   Value *Indices[] = {ConstantInt::get(Type::getInt32Ty(Context), 0),
     262           6 :                       ConstantInt::get(Type::getInt32Ty(Context), Idx),
     263          12 :                       ConstantInt::get(Type::getInt32Ty(Context), Idx2)};
     264           6 :   Value *Val = B.CreateGEP(Ty, BasePtr, Indices, Name);
     265             : 
     266             :   assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");
     267             : 
     268           6 :   return dyn_cast<GetElementPtrInst>(Val);
     269             : }
     270             : 
     271           5 : GetElementPtrInst *ShadowStackGCLowering::CreateGEP(LLVMContext &Context,
     272             :                                             IRBuilder<> &B, Type *Ty, Value *BasePtr,
     273             :                                             int Idx, const char *Name) {
     274           5 :   Value *Indices[] = {ConstantInt::get(Type::getInt32Ty(Context), 0),
     275           5 :                       ConstantInt::get(Type::getInt32Ty(Context), Idx)};
     276           5 :   Value *Val = B.CreateGEP(Ty, BasePtr, Indices, Name);
     277             : 
     278             :   assert(isa<GetElementPtrInst>(Val) && "Unexpected folded constant");
     279             : 
     280           5 :   return dyn_cast<GetElementPtrInst>(Val);
     281             : }
     282             : 
     283             : /// runOnFunction - Insert code to maintain the shadow stack.
     284      216012 : bool ShadowStackGCLowering::runOnFunction(Function &F) {
     285             :   // Quick exit for functions that do not use the shadow stack GC.
     286      432090 :   if (!F.hasGC() ||
     287      432090 :       F.getGC() != std::string("shadow-stack"))
     288             :     return false;
     289             :   
     290           2 :   LLVMContext &Context = F.getContext();
     291             : 
     292             :   // Find calls to llvm.gcroot.
     293           2 :   CollectRoots(F);
     294             : 
     295             :   // If there are no roots in this function, then there is no need to add a
     296             :   // stack map entry for it.
     297           2 :   if (Roots.empty())
     298             :     return false;
     299             : 
     300             :   // Build the constant map and figure the type of the shadow stack entry.
     301           2 :   Value *FrameMap = GetFrameMap(F);
     302           2 :   Type *ConcreteStackEntryTy = GetConcreteStackEntryType(F);
     303             : 
     304             :   // Build the shadow stack entry at the very start of the function.
     305             :   BasicBlock::iterator IP = F.getEntryBlock().begin();
     306           2 :   IRBuilder<> AtEntry(IP->getParent(), IP);
     307             : 
     308             :   Instruction *StackEntry =
     309           2 :       AtEntry.CreateAlloca(ConcreteStackEntryTy, nullptr, "gc_frame");
     310             : 
     311           4 :   while (isa<AllocaInst>(IP))
     312             :     ++IP;
     313           2 :   AtEntry.SetInsertPoint(IP->getParent(), IP);
     314             : 
     315             :   // Initialize the map pointer and load the current head of the shadow stack.
     316           2 :   Instruction *CurrentHead = AtEntry.CreateLoad(Head, "gc_currhead");
     317             :   Instruction *EntryMapPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
     318           2 :                                        StackEntry, 0, 1, "gc_frame.map");
     319           2 :   AtEntry.CreateStore(FrameMap, EntryMapPtr);
     320             : 
     321             :   // After all the allocas...
     322           7 :   for (unsigned I = 0, E = Roots.size(); I != E; ++I) {
     323             :     // For each root, find the corresponding slot in the aggregate...
     324           3 :     Value *SlotPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
     325           6 :                                StackEntry, 1 + I, "gc_root");
     326             : 
     327             :     // And use it in lieu of the alloca.
     328           6 :     AllocaInst *OriginalAlloca = Roots[I].second;
     329           3 :     SlotPtr->takeName(OriginalAlloca);
     330           3 :     OriginalAlloca->replaceAllUsesWith(SlotPtr);
     331             :   }
     332             : 
     333             :   // Move past the original stores inserted by GCStrategy::InitRoots. This isn't
     334             :   // really necessary (the collector would never see the intermediate state at
     335             :   // runtime), but it's nicer not to push the half-initialized entry onto the
     336             :   // shadow stack.
     337           4 :   while (isa<StoreInst>(IP))
     338             :     ++IP;
     339           2 :   AtEntry.SetInsertPoint(IP->getParent(), IP);
     340             : 
     341             :   // Push the entry onto the shadow stack.
     342             :   Instruction *EntryNextPtr = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
     343           2 :                                         StackEntry, 0, 0, "gc_frame.next");
     344             :   Instruction *NewHeadVal = CreateGEP(Context, AtEntry, ConcreteStackEntryTy,
     345           2 :                                       StackEntry, 0, "gc_newhead");
     346           2 :   AtEntry.CreateStore(CurrentHead, EntryNextPtr);
     347           2 :   AtEntry.CreateStore(NewHeadVal, Head);
     348             : 
     349             :   // For each instruction that escapes...
     350             :   EscapeEnumerator EE(F, "gc_cleanup");
     351           4 :   while (IRBuilder<> *AtExit = EE.Next()) {
     352             :     // Pop the entry from the shadow stack. Don't reuse CurrentHead from
     353             :     // AtEntry, since that would make the value live for the entire function.
     354             :     Instruction *EntryNextPtr2 =
     355             :         CreateGEP(Context, *AtExit, ConcreteStackEntryTy, StackEntry, 0, 0,
     356           2 :                   "gc_frame.next");
     357           2 :     Value *SavedHead = AtExit->CreateLoad(EntryNextPtr2, "gc_savedhead");
     358           2 :     AtExit->CreateStore(SavedHead, Head);
     359           2 :   }
     360             : 
     361             :   // Delete the original allocas (which are no longer used) and the intrinsic
     362             :   // calls (which are no longer valid). Doing this last avoids invalidating
     363             :   // iterators.
     364           7 :   for (unsigned I = 0, E = Roots.size(); I != E; ++I) {
     365           6 :     Roots[I].first->eraseFromParent();
     366           6 :     Roots[I].second->eraseFromParent();
     367             :   }
     368             : 
     369             :   Roots.clear();
     370             :   return true;
     371             : }

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